It sounded like an elaborate hoax. A petrol company, one of those evil pump-action purveyors of pure hydrocarbon, wanted to show me how to use less fuel by teaching me to drive in a more environmentally friendly manner! Thus reducing my carbon tyreprint and, bafflingly, their profits... It was an offer I couldn't ignore.

The company was BP and the challenge was to take their Green Driving Test, in which I would go as far as possible over a prepared course on exactly one litre of BP Ultimate unleaded petrol. There was no speed limit - rather than mph it was all about mpg. The car was a Ford S-Max fitted with a black box to record and analyse my second-by-second fuel consumption and exhaust emissions. For good measure, two BP technicians were also going to travel with me to observe my driving technique. I pointed out that the first thing I'd do as an economical driver would be to reduce the weight of my vehicle, but neither Alex nor Chris was prepared to get out.

An expert eco driver had already taken the test and thrown down the gauntlet by stretching the litre of petrol over eight miles. I edged the S-Max on to the course, driving as slowly and cautiously as possible, easing on the throttle as if I were walking on eggs. At 5.2 miles I ground to a halt. I was shocked at how badly I had done - 35 per cent less.

Back at base, Alex patiently talked me through my test. It's hard to get your head around just how much fuel a car uses. A full tank holds 60 litres of petrol - enough to fill a bath. To accelerate a car from standstill to 40mph burns around a cup of petrol. Try visualising that every time you pull away from the lights, and you'll feel your throttle foot getting lighter by the junction. Alex went on to explain that the secret of economical driving was maintaining momentum: 'Most cars weigh a couple of tonnes, and every time you touch the brakes you are wasting that rolling momentum. Imagine you are pushing a 2-tonne boulder. Getting it going is harder than keeping it going, and so it is with your car.'

Alex pointed out that I had only managed an average of 22 miles to the gallon, while the Green Driver had managed 34. The annual distance covered by British motorists is about 10,000 miles. At my rate of consumption that's £1,976 of petrol; eco man would only have spent £1,282. If saving the planet doesn't motivate you, saving your bank balance probably will.

Other tips for economising are to correctly inflate your tyres, maintain your car, remove your roof box and all excess weight (do you really need to keep your golf clubs in the boot?), turn off the aircon, turn off the engine rather than let it idle, smooth out stop-start motoring, avoid short journeys (cold engines use more fuel) and use a high-quality petrol... This last tip is the day's payoff. BP is in the business of selling fuel, after all. They just want you to buy BP Ultimate, which is more expensive. And why would you do that? Well, it comes with detergents which clean your engine as you use it, making your car more efficient, and cleaner. So much so, BP estimate you could get up to 28 miles more out of your tank. Couple that with your eco-driving skills, and you could start saving for an electric car.